Hitherto, the so-called openings such as windows of various buildings or vehicles are constructed with a transparent piece of glass or resin to allow sunlight through. However, sunlight contains ultraviolet light and infrared light in addition to visible light. Especially, near-infrared light between 800 nm to 2500 nm out of infra-red light are called heat waves, which cause the temperature inside a room to increase due to the heat waves coming inside from the opening.
Therefore, in recent years, a heat wave shielding material which blocks heat waves while still allowing a sufficient amount of visible light to enter and restrains a temperature increase inside a room while keeping brightness simultaneously, has been studied as a window material for various buildings and vehicles, and various steps are proposed for that purpose.
For instance, a heat wave shielding board in which a heat wave reflection film made by vapor deposition of metal on a transparent resin film is bonded to a transparent base material such as a glass board, an acrylic resin board, a polycarbonate resin board or the like is proposed in Japanese Patent Laid-open No. Sho 61-277437. However, since the heat reflection film itself is not only very expensive, but also requires complicated processing such as bonding and so on, this method has the disadvantage of being extremely costly. Further, since the bond between the transparent base material and the heat reflection film is not durable, peeling of the heat reflection film can occur over time.
Further, a number of heat wave shielding boards made by direct vapor deposition of metal or metal oxide on the surface of a transparent base material have been proposed. However, since these methods need a vapor deposition apparatus which requires a high vacuum or highly accurate environmental control, there arises problems that they suffer from poor mass productivity, lack of general versatility and, moreover, they result in heat wave shielding board that are very expensive.
The present inventors have proposed a coating solution for heat wave shielding made by allowing hexaboride fine particles alone, or hexaboride fine particles and ITO fine particles and/or ATO fine particles, as a heat wave shielding component, to be contained in various binders, and a heat wave shielding film obtained by coating the surface of a transparent base material with this coating solution and then hardening the coated material as in, for instance, Japanese Patent Laid-open No. Hei 11-181336, Japanese Patent Laid-open No. 2000-96034, Japanese Patent Laid-open No. 2000-169765, and so on.
As a means for shielding heat waves, in addition to the above-described method of applying a heat wave reflecting film or a heat wave shielding film on a transparent base material, a heat wave shielding board formed by incorporating mica covered with titanium oxide as heat wave reflection particles in a transparent resin such as acrylic resin or polycarbonate resin as in, for instance, Japanese Patent Laid-open No. Hei 5-78544 or Japanese Patent Laid-open No. Hei 2-173060 has been proposed.
However, this heat wave shielding board needs a large quantity of heat wave reflective particles to achieve good shieldability for heat waves, which causes a problem with visible light transmission. The amount of visible light allowed through decreases as the number of heat reflective particles is increased. Conversely, when the number of heat reflective particles is reduced, visible light transmission is enhanced, but heat wave shieldability is lowered. Therefore, it is difficult to satisfy heat wave shieldability and visible light transmission requirements at the same time. Further, the incorporation of large quantities of heat wave reflective particles weakens the transparent base resin, especially in terms of its impact resistance and toughness.
Considering the above-described drawbacks, an object of the present invention is to provide a heat wave shielding material which can be manufactured by a simple method without using a complicated manufacturing method or a high cost physical film-formation method, has excellent visible light transmission, exhibits high heat wave shieldability, and, furthermore, is excellent in strength such as impact resistance.